JP2000100470A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery having high safety and excellent in load characteristic. SOLUTION: In a nonaqueous electrolyte secondary battery provided with a positive electrode 1, a negative electrode 2 and a nonaqueous solvent electrolyte 4, the electrolyte 4 contains at least two of phosphate ester, a chain aprotic organic solvent having the boiling point of 25-150 deg.C and expressed by the composition formula Ca Hb XcSdOe, where X: halogen, 2<=a<=8, 0<=b<=a, 1<=c<=2a+2, 0<=d<=1, 1<=e<=4, and a chain compound having the boiling point of 25-150 deg.C and expressed by the composition formula CfHgXjN, where X: halogen, 3<=f<=8, O<=g<=f, 6<=j<=2f+2, The X in the composition formulas preferably contains fluorine of at least 50% or above.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly, to a non-aqueous electrolyte secondary battery having high safety and excellent load characteristics.

[0002]

2. Description of the Related Art Non-aqueous electrolyte secondary batteries represented by lithium secondary batteries have high capacity, high voltage and high energy density, and therefore, great expectations are placed on their development. .

However, such a non-aqueous electrolyte secondary battery uses an organic solvent as a constituent solvent of the electrolyte, and thus may cause a fire. That is, when the vapor pressure of the organic solvent in the electrolyte rises in the battery when the battery is exposed to a high temperature, and the electrolyte solvent reaching the flash point is released outside by the operation of the cleavage vent, the ignition source Ignition and ignition occur depending on the presence and the surrounding temperature. For this reason, current non-aqueous electrolyte secondary batteries generally meet certain safety standards by providing a cleavage vent, protection circuit, separator shutdown mechanism, etc.
The danger such as fire is higher than conventional water-based batteries.

[0004] As a method for solving such a problem, it has been studied to make the electrolyte solution nonflammable. Among them, phosphate esters are relatively promising, and it has been proposed to make the electrolyte solution flame-retardant using a chain phosphate triester (JP-A-7-114040, JP-A-8-228).
39, JP-A-8-111238, JP-A-8-111238
No. 321313).

[0005]

However, when chain phosphate triesters are added to the electrolytic solution, there is a problem that the load characteristics are reduced due to the high viscosity of the phosphate triesters. However, there is a problem that it cannot be used for a battery that requires a high output density.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a non-aqueous electrolyte secondary battery having high safety and excellent load characteristics by making the electrolyte flame-retardant while maintaining excellent load characteristics. And

[0007]

Means for Solving the Problems The present invention has been made as a result of intensive studies to solve the above problems,
A chain having a boiling point of 25 with the phosphoric acid ester in the electrolyte.
Composition formula C _a H _b X _c S _d O of not less than 150 ° C. and not more than 150 ° C.
_e (X: halogen, 2 ≦ a ≦ 8, 0 ≦ b ≦ a, 1 ≦ c ≦
2a + 2, non-brotonic organic solvent represented by 0 ≦ d ≦ 1, 1 ≦ e ≦ 4) or a composition formula C _f H _g X _j N (X: halogen, 3 ≦ f ≦ 8, 0 ≦ g ≦ f, By containing at least one compound represented by the formula (6 ≦ j ≦ 2f + 2), the flame resistance of the electrolytic solution is achieved while maintaining excellent load characteristics, and the safety and the load characteristics are high. It has been found that an excellent non-aqueous electrolyte secondary battery can be obtained.

As described above, it has been conventionally proposed to use a chain-like phosphate triester such as trimethyl phosphate as a solvent for an electrolytic solution. Although it acts as an agent and can make the electrolyte solution flame-retardant, there is a problem that the viscosity is high and the load characteristics are reduced.

Therefore, in the present invention, together with the phosphate ester, a chain formula having a boiling point of 25 ° C. or more and 150 ° C. or less, C _a H _b X _c S _d O _e (X: halogen, 2 ≦ a ≦ 8, 0
≦ b ≦ a, 1 ≦ c ≦ 2a + 2, 0 ≦ d ≦ 1, 1 ≦ e ≦
Aprotic organic solvent represented by 4) or a composition formula C _f
H _g X _j N (X: halogen, 3 ≦ f ≦ 8, 0 ≦ g ≦ f,
It has been found that by using at least one compound represented by the formula (6 ≦ j ≦ 2f + 2), the load characteristics can be improved while maintaining the flame retardancy of the electrolytic solution.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Composition formula C _a used in the present invention
As belonging to the aprotic organic solvent represented by H _b X _c S _d O _e, for example, ethers of alkyl halides, thioether, carboxylic acid esters, carboxylic acid halides, carboxylic acid anhydrides, sulfonic acid ester , Sulfonic acid halides, sulfonic anhydrides, and the like, but are not limited thereto.

Examples of the compound represented by the composition formula C _f H _g X _j N include, but are not limited to, trifluoroalkylamines.

The above aprotic organic solvents and compounds are
In general, the induction rate is low, but it is flame retardant and has a lower viscosity than phosphate ester, so when used in combination with phosphate ester, it compensates for each other's drawbacks while maintaining the flame retardancy of the electrolyte. The electric conductivity of the electrolyte can be improved. These aprotic organic solvents and compounds may be used alone or in combination of two or more. X in the composition formula represents any halogen element, but considering the effect on the environment, 50% of X (however,
(% Of the number of atoms) or more is preferably fluorine, and more preferably all fluorine.

In order for the phosphate ester to be flame-retardant, it preferably has 6 or less carbon atoms, more preferably 4 or less carbon atoms. Further, a phosphate having a cyclic structure having -OPO- as a part of the ring structure has a high induction ratio and is suitably used in the present invention. Examples of the phosphate having such a cyclic structure include methyl ethylene phosphate and ethyl ethylene phosphate. Examples of the acyclic phosphate include trimethyl phosphate, triethyl phosphate, and the like. Of course, these can be used in combination. Further, the phosphoric acid ester may have a halogenated alkyl group, an unsaturated hydrocarbon group, a carboxylic acid ester group, a carbonate ester group, or the like in the structure. When the amount of the phosphate ester in the electrolyte is small, the flame retardancy may not be sufficiently exhibited, and when the amount is too large, the load characteristics may be reduced. In the above, the volume ratio is preferably 20% or more, more preferably 50% or more, and preferably 90% or less, more preferably 80% or less. Electrolyte is composed by dissolving the electrolyte in the electrolyte solvent,
Since the volume of the electrolyte at the time of dissolution is much smaller than the volume of the electrolyte solvent, the volume of the electrolyte can be regarded as substantially the same as the volume of the electrolyte solvent.

The aprotic organic solvent represented by the above composition formula C _a H _b X _c S _d O _e or the compound represented by the composition formula C _f H _g X _j N is 5% by volume in the electrolytic solution. % Or more, more preferably 10% or more,
It is preferably at most 50%, more preferably at most 20%.

The composition formula C _{a Hb Xc} used in the present invention.
An aprotic organic solvent represented by S _d O _e or a composition formula C _f
The compound represented by H _g X _j N needs to have a chain shape and a boiling point of 25 ° C. or more and 150 ° C. or less, which is based on the following reason. That is, since the aprotic organic solvent or compound is in a chain form, the viscosity becomes low,
Further, when the boiling point is 25 ° C. or higher, it is easy to handle, and when the boiling point is 150 ° C. or lower, it does not become too high in viscosity and is easy to handle.

Further, depending on the type of the negative electrode active material, the phosphate ester may cause a decomposition reaction on the surface of the negative electrode. Even in such a case, the decomposition is caused by including a cyclic carbonate or a carbonate multimer in the electrolytic solution. Can be suppressed. In order to maintain the flame retardancy of the electrolytic solution, the mixing amount of such carbonates is preferably 20% or less by volume in the electrolytic solution, and is preferably 10% or less.
The following is more preferred.

As the above cyclic carbonate, for example,
Examples include ethylene carbonate, propylene carbonate, and vinylene carbonate. Examples of the carbonate polymer include a compound represented by the general formula: R ₁ OC (O) OR ₂ OC
(O) OR ₃ (where R ₁ and R ₃ are an alkyl group having 1 to 4 carbon atoms; R ₂ is an alkyl group having 2 to 4 carbon atoms;
₁ and R ₃ may be the same or different)
And specifically, for example, 1,
2-bismethoxycarbonyloxyethylene, 1,2-
Examples include bismethoxycarbonyloxypropylene, 1,2-bisethoxycarbonyloxyethylene, and 1,2-bisethoxycarbonyloxypropylene.

In the present invention, in addition to the above-mentioned organic solvents, for example, 1,2-dimethoxyethane,
One or more of 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 1,3-dioxolane, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate and the like can be used in combination, but the flame retardancy of the electrolytic solution can be used. In order to maintain the properties, these are preferably 20% or less by volume in the electrolytic solution, and 10% or less.
The following is more preferred.

As the electrolyte, for example, LiClO_Four,
LiPF₆, LiBF_Four, LiAsF₆, LiSb
F₆, LiCF_ThreeSO_Three, LiC_FourF₉SO_Three, LiC
F_ThreeCO _Two, Li_TwoC_TwoF_Four(SO_Three)_Two, LiN (C
F_ThreeSO_Two)_Two, LiC (CF_ThreeSO_Two)_Three, LiC_n
F_{2n + 1}SO_Three(N ≧ 2), LiN (RfO SO_Two)_Two
[Where Rf is a fluoroalkyl group] alone or
Can be used as a mixture of two or more.
F₆And LiC_FourF₉SO_ThreeAre preferred. In the electrolyte
Electrolyte concentration is not particularly limited
However, if the concentration is higher than 1 mol / l, safety will increase.
It is preferable because it is even better, and it is preferably at least 1.2 mol / l.
Is more preferable. In addition, the concentration of the electrolyte in the electrolyte is
When the content is less than 1.7 mol / l, the electric characteristics are improved.
Preferably, it is 1.5 mol / l or less.

In the present invention, as the positive electrode active material, for example, lithium cobalt oxide such as LiCoO ₂ ;
lithium nickel oxide such as iNiO ₂ , LiMn ₂
Lithium manganese oxides such as O ₄ , lithium titanium oxides such as LiTiO ₂ , and metals such as Li (NiCo) O ₂ in which a part of Ni of LiNiO ₂ is replaced by Co, manganese dioxide, vanadium pentoxide, and chromium oxide Oxides or metal sulfides such as titanium disulfide and molybdenum disulfide are used. Among them, LiNiO ₂ , Li
The open circuit voltage during charging of NiO ₂ , LiMn ₂ O _4, etc. is L
A lithium composite oxide exhibiting 4 V or more on an i basis is preferable because a high energy density can be obtained. Further, when the present invention is applied to a case where a thermally unstable lithium nickel oxide such as LiNiO ₂ or a composite oxide containing nickel is used as the positive electrode active material, the effect is remarkably exhibited.

For the positive electrode, for example, a conductive additive such as flake graphite or a binder such as polyvinylidene fluoride is added to the above-mentioned positive electrode active material, if necessary, and mixed to prepare a positive electrode mixture. Is dissolved or dispersed in water or a solvent to form a paste (the binder may be dissolved in a solvent or the like in advance and then mixed with the positive electrode active material or the like),
The positive electrode mixture paste is applied to a current collector made of aluminum foil or the like, dried, and formed through a step of forming a positive electrode mixture layer on at least one surface of the current collector. However, the method for producing the positive electrode is not limited to the method described above, but may be another method.

In the present invention, the active material of the negative electrode includes:
Any material capable of doping and undoping lithium ions may be used. Examples of such a negative electrode active material include graphite,
Carbon compounds such as pyrolytic carbons, cokes, glassy carbons, fired bodies of organic polymer compounds, mesocarbon microbeads, carbon fibers, and activated carbon are exemplified. Also, S
Some compounds such as alloys such as i, Sn, and In and oxides that can be charged and discharged at a low potential close to Li have higher capacities than carbon materials, and they are also suitably used.

The negative electrode is made of, for example, a conductive auxiliary such as flake graphite, carbon black, or the like, or polyvinylidene fluoride, polytetrafluoroethylene, ethylene propylene diene rubber, or polyacrylic, if necessary. A binder such as an acid and carboxymethyl cellulose is added and mixed to prepare a negative electrode mixture, which is dissolved or dispersed in water or a solvent to form a paste (the binder is first dissolved in a solvent or the like, and then the negative electrode active material is prepared). And the like, and the negative electrode mixture paste is applied to a current collector made of copper foil or the like, dried, and subjected to a step of forming a negative electrode mixture layer on at least one surface of the current collector. It is produced by However, the method for producing the negative electrode is not limited to the method exemplified above, and may be another method.

As the current collector for the positive electrode and the negative electrode, for example, a metal foil such as aluminum, copper, nickel, and stainless steel, or a net formed of these metals is used. Aluminum foil is particularly suitable, and copper foil is particularly suitable as the negative electrode current collector.

As the separator, any separator having sufficient strength and capable of holding a large amount of electrolyte may be used. From such a viewpoint, the thickness is 10 to 50 μm and the porosity is 30 to 70 μm.
% Of polyethylene, polypropylene, or a copolymer of ethylene and propylene.

The battery is, for example, a spirally wound electrode body or a laminated electrode body such as a spirally wound electrode body produced by spirally winding a separator between the positive electrode and the negative electrode produced as described above. Is inserted into a nickel-plated iron or stainless steel battery case and sealed. Further, the battery usually incorporates an explosion-proof mechanism for discharging the gas generated inside the battery to the outside of the battery when the pressure has risen to a certain pressure, thereby preventing the battery from bursting under high pressure.

[0027]

Next, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to only these examples.

Example 1 Trimethyl phosphate, methyl heptafluorobutanoate (CF ₃ CF ₂ COOCH ₃ ) and ethylene carbonate were mixed at a volume ratio of 85: 10: 5, and 1.0 mol of LiPF ₆ was added to the mixed solvent. / L was dissolved to prepare a non-aqueous solvent-based electrolyte solution.

Separately from the above, flaky graphite was added to LiNiO ₂ as a conductive additive at a weight ratio of 100: 7 and mixed, and this mixture was mixed with a solution obtained by dissolving polyvinylidene fluoride in N-methylpyrrolidone. Was mixed into a paste. This positive electrode mixture paste is uniformly applied to both surfaces of a positive electrode current collector made of aluminum foil having a thickness of 20 μm, dried to form a positive electrode mixture layer, compression-molded by a roller press, and cut. The lead body was welded to produce a strip-shaped positive electrode.

Next, a non-graphitizable carbon material (coke) and a solution of polyvinylidene fluoride dissolved in N-methylpyrrolidone were mixed to prepare a negative electrode mixture paste. The negative electrode mixture paste was uniformly applied on both surfaces of a negative electrode current collector made of a strip-shaped copper foil having a thickness of 10 μm, dried to form a negative electrode mixture layer, compression-molded by a roller press, and cut. Thereafter, the lead body was welded to produce a strip-shaped negative electrode.

The above-mentioned strip-shaped positive electrode is overlaid on the above-mentioned strip-shaped negative electrode via a microporous polyethylene film having a thickness of 25 μm,
After spirally winding into a spiral electrode body, the outer diameter is 14 m.
m, and the positive electrode and the negative electrode were welded together. Next, the electrolytic solution 1.
After pouring 6 ml into the battery case and allowing the electrolyte to sufficiently penetrate into the separator and the like, sealing, pre-charging and aging, the cylindrical non-aqueous electrolyte having the structure as shown in the schematic diagram of FIG. A secondary battery was manufactured.

Referring to the battery shown in FIG. 1, 1 is the positive electrode and 2 is the negative electrode. In FIG. 1, in order to avoid complication, the current collector and the negative electrode used in producing the positive electrode 1 and the negative electrode 2 were used. A metal foil or the like as a base is not shown.
The positive electrode 1 and the negative electrode 2 are spirally wound with a separator 3 interposed therebetween, and are housed in a battery case 5 together with the electrolytic solution 4 as a spiral electrode body.

The battery case 5 is made of stainless steel, and an insulator 6 made of polypropylene is disposed at the bottom of the battery case 5 before the spiral electrode body is inserted. The sealing plate 7 is made of aluminum, has a disk shape, and has a thin portion 7 in the center.
In addition, a hole is provided around the thin portion 7a as a pressure introduction port 7b for applying the internal pressure of the battery to the explosion-proof valve 9. The projection 9a of the explosion-proof valve 9 is welded to the upper surface of the thin portion 7a to form a welded portion 11. In addition, the thin portion 7a provided on the sealing plate 7 and the protruding portion 9a of the explosion-proof valve 9 are illustrated only in a cut plane so as to be easily understood in the drawings, and a contour line behind the cut plane is shown. Is not shown. Further, the welded portion 11 between the thin portion 7a of the sealing plate 7 and the protruding portion 9a of the explosion-proof valve 9 is shown in an exaggerated state rather than the actual one so as to be easily understood in the drawings.

The terminal plate 8 is made of rolled steel, nickel-plated on its surface, and has a hat-like shape with a peripheral edge formed in a flange shape. The terminal plate 8 is provided with a gas outlet 8a. . The explosion-proof valve 9 is made of aluminum and has a disk shape.
In the center thereof, a protruding portion 9a having a tip portion is provided on the power generation element side (the lower side in FIG. 1), and a thin portion 9b is provided, and the lower surface of the protruding portion 9a is, as described above, It is welded to the upper surface of the thin portion 7a of the sealing plate 7 to form a welded portion 11. The insulating packing 10 is made of polypropylene and has an annular shape. The insulating packing 10 is disposed above the peripheral portion of the sealing plate 7, and the explosion-proof valve 9 is disposed above the insulating packing 10. The insulating packing 10 insulates the sealing plate 7 from the explosion-proof valve 9. The gap between the two is sealed so that the electrolyte does not leak from between the two. The annular gasket 12 is made of polypropylene, and the lead body 13 is made of aluminum. The sealing plate 7 and the positive electrode 1 are connected to each other. An insulator 14 is disposed above the spiral electrode body. The bottom portion is connected by a lead body 15 made of nickel.

In this battery, the thin portion 7 of the sealing body 7
a and the explosion-proof valve 9a come into contact at the welded portion 11, and the explosion-proof valve 9a
And the peripheral edge of the terminal plate 8 are in contact with each other, and the positive electrode 1 and the sealing body 7 are connected by the lead 13 on the positive electrode side.
The positive electrode 1 and the terminal plate 8 are electrically connected to each other by the lead body 13, the sealing body 7, the explosion-proof valve 9, and the welded portion 11 thereof, and function normally as an electric circuit.

When an abnormal situation occurs in the battery and gas is generated inside the battery and the internal pressure of the battery rises, the internal pressure rises and the central part of the explosion-proof valve 9 moves in the direction of the internal pressure (FIG. 1).
Then, it is deformed in the upper direction)
The shearing force acts on the thin portion 7a integrated at 1 and the thin portion 7a is broken or the projection 9a of the explosion-proof valve 9 is formed.
And the thin portion 7a of the sealing plate 7 is peeled off, whereby the electrical connection between the positive electrode 1 and the terminal plate 8 is lost and the current is cut off. As a result, the battery reaction does not proceed, so even during overcharge or short circuit, the battery temperature and internal pressure rise due to the charging current and short circuit current do not progress further, preventing the battery from bursting under high pressure Designed to be able to.

Example 2 Example 2 was repeated except that a mixed solvent of methyl ethylene phosphate, perfluorotriethylamine and 1,2-bisethoxycarboxyoxyethylene in a volume ratio of 85: 10: 5 was used as a solvent for the electrolytic solution. A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1.

Comparative Example 1 A non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1, except that a mixed solvent of trimethyl phosphate and ethylene carbonate in a volume ratio of 95: 5 was used as a solvent for the electrolyte. did.

Comparative Example 2 The procedure of Example 1 was repeated except that a mixed solvent of methyl ethylene phosphate and 1,2-bisethoxycarboxyoxyethylene in a volume ratio of 95: 5 was used as a solvent for the electrolytic solution. A water electrolyte secondary battery was produced.

Comparative Example 3 A non-aqueous electrolyte secondary battery was prepared in the same manner as in Example 1, except that a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 50:50 was used as a solvent for the electrolyte. did.

The batteries of Examples 1 and 2 and Comparative Examples 1 and 2 were examined for load characteristics, and the batteries of Examples 1 and 2 and Comparative Examples 1 to 3 were subjected to flammability tests.
The test methods and evaluation results are as follows.

Load characteristics: Examples 1-2 and Comparative Examples 1
The battery of No. 2 was charged and discharged at a voltage in the range of 2.75 V to 4.1 V, and the current density of the discharge capacity at a current density of 1 C was 0.1.
The ratio to the discharge capacity at 2C was examined. The results are shown in Table 1 as percentages as "load characteristics (1C / 0.2C)".

[0043]

[Table 1]

Flammability test: Examples 1-2 and Comparative Example 1
For the batteries of Nos. 1 to 3, the battery was heated to a high temperature and the safety valve was operated [that is, in the battery shown in FIG.
Gas is generated inside the battery due to evaporation of the solvent from the electrolytic solution and the like. When the internal pressure of the battery rises and reaches a predetermined pressure, the internal pressure rises and the central part of the explosion-proof valve 9 moves in the direction of the internal pressure (see FIG. In FIG. 1, the thin portion 7a integrated at the welded portion 11 is subjected to a shearing force, and the thin portion 7a is broken or the projection 9a of the explosion-proof valve 9 is deformed. And the thin portion 7a of the sealing plate 7 is peeled off, whereby the gas inside the battery is discharged from the gas discharge port 8a of the terminal plate 8 to the outside of the battery. The sealing portion on the upper part of the battery was disassembled in advance to expose the inside of the battery, and in this state, the battery was heated to 100 ° C., and a fire was brought close to the exposed portion to determine whether or not ignition occurred.

[0045]

[Table 2]

As is clear from the results shown in Table 2, the batteries of Examples 1 and 2 were safer than the batteries of Comparative Example 3 which did not use a phosphate such as trimethyl phosphate or methyl ethyl phosphate. As is clear from the results shown in Table 1, the batteries of Examples 1 and 2 had better load characteristics than the corresponding batteries of Comparative Examples 1 and 2, respectively.

[0047]

As described above, according to the present invention, a non-aqueous electrolyte secondary battery having high safety and excellent load characteristics can be provided.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view schematically showing one example of a non-aqueous electrolyte secondary battery according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Separator 4 Electrolyte

Claims (5)

[Claims] 1. A non-aqueous electrolyte secondary battery having a positive electrode, a negative electrode, and a non-aqueous solvent-based electrolyte, wherein the electrolyte comprises:
A phosphoric acid ester, and a chain formula having a boiling point of 25 ° C. or more and 150 ° C. or less, C _a H _b X _c S _d O _e (X: halogen, 2 ≦
a ≦ 8, 0 ≦ b ≦ a, 1 ≦ c ≦ 2a + 2, 0 ≦ d ≦ 1,
An aprotic organic solvent represented by 1 ≦ e ≦ 4) or a chain-form composition formula C _f H _g X having a boiling point of from 25 ° C. to 150 ° C.
_j N (X: halogen, 3 ≦ f ≦ 8, 0 ≦ g ≦ f, 6 ≦ j
≦ 2f + 2). A non-aqueous electrolyte secondary battery comprising at least two of the following compounds: 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the phosphate ester is trimethyl phosphate. 3. The phosphate ester according to claim 1, wherein the phosphate ester has a cyclic structure having -OPO- as a part of the ring.
The non-aqueous electrolyte secondary battery according to the above. 4. The non-aqueous electrolyte secondary battery according to claim 1, wherein at least 50% or more of X is fluorine. 5. The non-aqueous electrolyte secondary battery according to claim 1, wherein the electrolyte contains 20% or less by volume of a cyclic carbonate or a carbonate multimer.